1. Field of the Invention
The present invention relates generally to the production of antigen-binding molecules. More specifically, the invention relates to multivalent forms of antigen-binding proteins. Compositions of, genetic constructions for, methods of use, and methods for producing these multivalent antigen-binding proteins are disclosed.
2. Description of the Background Art
Antibodies are proteins generated by the immune system to provide a specific molecule capable of complexing with an invading molecule, termed an antigen. FIG. 14 shows the structure of a typical antibody molecule. Natural antibodies have two identical antigen-binding sites, both of which are specific to a particular antigen. The antibody molecule "recognizes" the antigen by complexing its antigen-binding sites with areas of the antigen termed epitopes. The epitopes fit into the conformational architecture of the antigen-binding sites of the antibody, enabling the antibody to bind to the antigen.
The antibody molecule is composed of two identical heavy and two identical light polypeptide chains, held together by interchain disulfide bonds (see FIG. 14). The remainder of this discussion will refer only to one light/heavy pair of chains, as each light/heavy pair is identical. Each individual light and heavy chain folds into regions of approximately 110 amino acids, assuming a conserved three-dimensional conformation. The light chain comprises one variable region (termed V.sub.L) and one constant region (C.sub.L), while the heavy chain comprises one variable region (V.sub.H) and three constant regions (C.sub.H 1, C.sub.H 2 and C.sub.H 3). Pairs of regions associate to form discrete structures as shown in FIG. 14. In particular, the light and heavy chain variable regions, V.sub.L and V.sub.H, associate to form an "F.sub.V " area which contains the antigen-binding site.
The variable regions of both heavy and light chains show considerable variability in structure and amino acid composition from one antibody molecule to another, whereas the constant regions show little variability. The term "variable" as used in this specification refers to the diverse nature of the amino acid sequences of the antibody heavy and light chain variable regions. Each antibody recognizes and binds antigen through the binding site defined by the association of the heavy and light chain variable regions into an F.sub.V area. The light-chain variable region V.sub.L and the heavy-chain variable region V.sub.H of a particular antibody molecule have specific amino acid sequences that allow the antigen-binding site to assume a conformation that binds to the antigen epitope recognized by that particular antibody.
Within the variable regions are found regions in which the amino acid sequence is extremely variable from one antibody to another. Three of these so-called "hypervariable" regions or "complementarity-determining regions" (CDR's) are found in each of the light and heavy chains. The three CDR's from a light chain and the three CDR's from a corresponding heavy chain form the antigen-binding site.
Cleavage of the naturally-occurring antibody molecule with the proteolytic enzyme papain generates fragments which retain their antigen-binding site. These fragments, commonly known as Fab's (for Fragment, antigen binding site) are composed of the C.sub.L, V.sub.L, C.sub.H 1 and V.sub.H regions of the antibody. In the Fab the light chain and the fragment of the heavy chain are covalently linked by a disulfide linkage.
Recent advances in immunobiology, recombinant DNA technology, and computer science have allowed the creation of single polypeptide chain molecules that bind antigen. These single-chain antigen-binding molecules incorporate a linker polypeptide to bridge the individual variable regions, V.sub.L and V.sub.H, into a single polypeptide chain. A computer-assisted method for linker design is described more particularly in U.S. Pat. No. 4,704,692, issued to Ladner et al. in November, 1987, and incorporated herein by reference. A description of the theory and production of single-chain antigen-binding proteins is found in U.S. Pat. No. 4,946,778 (Ladner et al.), issued Aug. 7, 1990, and incorporated herein by reference. The single-chain antigen-binding proteins produced under the process recited in U.S. Pat. No. 4,946,778 have binding specificity and affinity substantially similar to that of the corresponding Fab fragment.
Bifunctional, or bispecific, antibodies have antigen binding sites of different specificities. Bispecific antibodies have been generated to deliver cells, cytotoxins, or drugs to specific sites. An important use has been to deliver host cytotoxic cells, such as natural killer or cytotoxic T cells, to specific cellular targets. (U. D. Staerz, O. Kanagawa, M. J. Bevan, Nature 314:628 (1985); S. Songilvilai, P. J. Lachmann, Clin. Exp. Immunol. 79: 315 (1990)). Another important use has been to deliver cytotoxic proteins to specific cellular targets. (V. Raso, T. Griffin, Cancer Res. 41:2073 (1981); S. Honda, Y. Ichimori, S. Iwasa, Cytotechnology 4:59 (1990)). Another important use has been to deliver anti-cancer non-protein drugs to specific cellular targets (J. Corvalan, W. Smith, V. Gore, Intl. J. Cancer Suppl. 2:22 (1988); M. Pimm et al., British J. of Cancer 61:508 (1990)). Such bispecific antibodies have been prepared by chemical cross-linking (M. Brennan et al., Science 229:81 (1985)), disulfide exchange, or the production of hybrid-hybridomas (quadromas). Quadromas are constructed by fusing hybridomas that secrete two different types of antibodies against two different antigens (Kurokawa, T. et al., Biotechnology 7.1163 (1989)).